Capturing carbon dioxide gas and storing it underground is a promising strategy for reducing greenhouse gases in the atmosphere. Scientists at the National Institute of Standards and Technology (NIST) have taken an important first step in validating a technique for monitoring carbon dioxide emissions from sequestration sites.
The NIST team members, who collaborated on the study with Harris Corporation and Atmospheric and Environmental Research (AER), said the findings could permit far more effective monitoring of sequestration sites under real-world conditions, which ordinarily make it difficult to determine whether the carbon dioxide is escaping storage.
Carbon sequestration involves removing carbon dioxide gas from power plant smokestack streams and other large emission sources that release greenhouse gases into the atmosphere. Once captured, the carbon dioxide can be pumped deep into the earth, effectively removing it from the atmosphere.
A sticking point is that the carbon dioxide must remain underground for centuries. If more than 0.1 percent of the gas leaks out per year, it's all for nothing. So scientists from around the world have been trying to develop an effective way to monitor sites for potential gas leaks.
One approach places a system of laser reflectors above the ground directly over a carbon storage site to scan for escaping gas. Traditional methods of scanning the region with a laser can reveal leaks. Collecting useful data, however, requires a half-hour period when the wind does not shift and the sampled atmosphere does not change. This isn't a common situation, and if the wind shifts, the data is ruined.
Enter the NIST-Harris-AER team. Harris and AER built the laser-based measurement system under a cooperative agreement with the Department of Energy's National Energy Technology Laboratory. The system collected data over a mock storage site in Ft. Wayne, Indiana. The data went to NIST for analysis. NIST developed a mathematical model that considers the change in shape of a gas leakage plume in the wind. In this way, the model factors out other carbon dioxide sources in the sequestration area. The practical upshot is that wind variables and other outdoor field conditions are no longer a constraint.
The results from their analysis surprised them, and not just because of the pleasing findings: They could now, according to their simulations, pinpoint a gas leak from the ground to within about 5 meters, or ten times more accurately than other approaches, and regardless of the wind conditions.
"What surprised us is that even though one of these experiments was meant to be one with no carbon source present, we found one anyway," said NIST physicist Zachary Levine. "This was supposed to be the 'null set' that we were going to compare with data from another field that has an artificial carbon dioxide source buried beneath it. Instead, we found something none of us had expected."
Levine described the results as a proof of concept for carbon sequestration monitoring.
"The approach means far fewer demands placed on the laser sensors, and much reduced worries about unrealistic wind conditions," he said. "It also means we can detect less intense leaks with far better spatial resolution. We're looking forward to testing it further on additional field campaigns."
Z.H. Levine, A.L. Pintar, J.T. Dobler, N. Blume, M. Braun, T.S. Zaccheo and T.G. Pernini. The detection of carbon dioxide leaks using quasi-tomographic laser absorption spectroscopy measurements in variable wind. Atmospheric Measurement Techniques, http://www.atmos-meas-tech.net/9/1627/2016/, April 13, 2016.